1. Field of the Invention
This invention relates to a presensitized planographic printing plate which can be used both in negative and positive work and does not require dampening water during printing, and more specifically to a presensitized planographic printing plate comprising a combination of a photosensitive substance and a silicone rubber on a support, which can be used both in negative and positive work by changing the method of treatment after exposure.
2. Description of the Prior Art
The term "positive work" refers to printing on a presensitized plate using a positive film, and the term "negative work" refers to printing on a presensitized plate using a negative film.
In the preparation of planographic printing plates used mainly for offset printing, a photomechanically producible presensitized plate is exposed through a negative or positive film, and treated with a suitable developer. This treatment results in the imagewise formation of a hydrophilic portion and an oleophilic portion to yield a planographic printing plate which can be readily used. The olephilic portion accepts oily inks, and an image area is formed at the portion which is susceptible to ink adhesion. On the other hand, the hydrophilic portion repels oily inks, and a non-image area is formed at the portion to which the ink has not adhered. This is the essential concept of printing plates of this type. However, the ink receptivity of the oleophilic portion is not compatible with the ink repelling property of the hydrophilic portion. Accordingly, it is necessary to increase the ink-repelling property of the hydrophilic portion during printing by alternately supplying dampening water and oily ink.
This dampening operation has been one of the great defects of conventional offset planographic printing plates. It has long been desired in the art of offset planographic printing to remove the defects which accompany the use of dampening water such as the need for expert skill in balancing the supply of dampening water against that of the oily ink, the difficulty of producing printing inks not miscible with dampening water and the elongation or shrinkage of the printing plate.
In an attempt to remove these defects, methods were developed in which a silicone rubber layer which strongly repels ink was formed on the surface of the printing plate, thereby making it possible to carry out printing only by the supply of oily ink without the use of dampening water. For example, the Japanese language publication "Journal of Printing," Vol. 53, Oct. 19770, page 48 and November 1970, page 51, describes presensitized dry plates which do not require dampening water. Japanese Pat. Nos. 23042/69 and 16044/71, and U.S. Pat. No. 3,511,178 also disclose the production of presensitized printing plates which do not require dampening water.
The method of producing such presensitized plates generally comprises forming a silicone rubber layer on top of a photosensitive layer formed on a support, imagewise exposing the resulting structure, and then treating the exposed structure to remove the silicone rubber layer at the image areas while leaving it at the non-image areas. When the resulting printing plate is inked, the ink does not adhere to the non-image areas (where the silicone rubber remains) because the cohesive force of the ink is larger than the force of adherance between the ink and the silicone rubber. On the other hand, the ink adheres to the image areas which are free of the silicone rubber. The ink which has adhered only to the image areas is transferred to printing paper, and printed copies can be obtained. Since the silicone rubber layer has a sufficiently high ink repelling property, it is not necessary to supply dampening water as with conventional presensitized plates.